A chain gear is usually made annular, has at its outer periphery a number of teeth, and is connected to a crank through an adapter, so that a torque from pedalling is transmitted from the crank to the chain gear through stays of the adapter.
The adapter has five stays disposed at about equal intervals circumferentially of the chain gear. The chain gear is supported to the stays and as a whole has a cross-sectional area greater than a minimum standard area which is required to withstand the maximum torque acting on the crank arms, thereby preventing a breakdown of the chain gear. In other words, the chain gear usually has five or more stays and the greater than minimum cross-sectional area because it has generally been considered that less than five stays and a reduced cross-sectional area of the chain gear are undesirable from the standpoint of strength. Therefore, a problem is created that the gear crank as a whole has an increased weight.
A bottom bracket at the bicycle frame supports a seat tube, a down tube, and a chain stay as well as the gear crank and extends at one axial end axially outwardly from the mounting portion for both the tubes so as to carry the gear crank. Therefore, during the bicycle's running, air flows to strike the front side of the axial end of the bottom bracket at the chain gear side to thereby cause an eddy flow in a space surrounded by the front side, down tube, and chain gear, the eddy flow creating a large air resistance against the bicycle's running. This not only deteriorates the running quality of the bicycle at high speed, but also increases driver fatigue.